Frequent colds when you train hard: what your blood says about it

Getting frequent colds is less unusual for most adults than it feels: two to four a year is ordinary, and the chance of a respiratory infection peaks in winter. If you train seriously, a second question follows, namely whether your training load itself plays a part.

The answer is more nuanced than the familiar diagrams suggest.

What bothers me about this topic: everywhere you read the same advice about sleep, hand washing and vitamin C, and nowhere anything about what your blood count can and cannot say. That is the question athletes ask most.

How many colds a year is normal?

Two to four colds a year is ordinary for adults. People with young children usually sit higher, simply through exposure at school and nursery. RIVM figures on respiratory infections also show a clear seasonal pattern, peaking in the cold months.

A cold lasts seven to ten days on average. If you catch three back to back with short gaps, it feels like one long stretch of illness, while statistically it is three.

That distinction is not academic. People who say they were ill all winter often turn out to sit inside the ordinary range once they count the separate episodes.

Does hard training make you catch colds more often?

Probably less often than was long assumed. The classic model is the J-curve: moderate exercise supports your immune system, very hard exercise suppresses it temporarily, with an open window of a few hours in which you are more susceptible. That second part has come under serious challenge in recent years.

A detailed review calls the classic explanation a myth. The reasoning: the fall in lymphocytes after exercise is not a loss but a redistribution to the lungs, gut and skin, where those cells stand guard (PMID 29713319). Follow-up work reaches the same conclusion (PMID 32139352).

That does not mean training load is irrelevant. A study linking training load to respiratory infections did find a relationship between load and the number of infection days (PMID 22151281). Research on immune regulation during exercise describes a similarly nuanced picture (PMID 26477922).

My reading: yesterday's hard session is not what makes you ill, but stacking months without recovery raises the odds. That is a different problem, with a different solution.

What else can be behind it?

Exposure is the biggest factor, and it has nothing to do with your immune system. Beyond that, sleep debt, prolonged stress, a chronically low energy intake and smoking play a part. In a small share of people there is a medical cause underneath, such as a deficiency affecting the production of immune cells.

The bottom row is the only one that calls for a doctor. The rest is context you can act on yourself.

What do your blood values show if you get frequent colds?

Usually nothing remarkable, and that is informative in itself. A leukocyte differential shows how your white cells are distributed at that moment. It is a snapshot of your immune system, not a measure of how strong it is.

Imagine two athletes who both had 4 colds this winter and both show lymphocytes at 1.6 x10⁹/l with a CRP of 2 mg/l, neatly inside the reference ranges. The first sleeps six hours and is four months into a build block. The second sleeps eight hours and has two children in primary school.

The same blood count, two very different explanations. For one it points to load and sleep, for the other to exposure.

So a normal result is not a useless one. You ruled out what you wanted to rule out, and that moves the conversation to the things you can actually steer.

Measure when you have been symptom-free for a few weeks where you can. Draw mid-cold and you measure the cold.

How do you catch fewer colds?

There is no reliable switch for this, and anyone promising otherwise is selling you something. What research does describe is association: people who sleep enough, build their training load gradually and eat enough for what they do report fewer infection days on average.

Reducing exposure helps most and is the least interesting thing to hear. Hand hygiene in a busy gym does more than any supplement.

On supplements: take that to your GP or a dietitian rather than a forum. Whether something is useful for you depends on your own situation, and I cannot fill that in for you here.

When should you discuss it with your GP?

If your infections run unusually severe or unusually long, if you have needed antibiotics for respiratory infections, or if there are symptoms that stand apart from them. Persistent fever, unexplained weight loss, night sweats or stubborn fatigue belong in a conversation.

Thuisarts describes recurrent infections as a reason to look further when the pattern differs from what is usual. The NHG guidelines approach it the same way. Your GP can help you decide whether further steps are needed.

Take to that conversation how often and how long you were ill, and whether you had a fever. That is more useful than the feeling that you were ill all winter.

Where do you start?

Count your episodes first, and measure after. Four short colds in a winter is a different thing from three unbroken months of symptoms. If you want to rule out something in your blood count, our complete blood count test splits your white series out, with an assessment by a BIG-registered doctor.

How to read those values is in complete blood count explained. If your immune cells drop after a hard week, see low lymphocyte count. For the cells that respond to bacteria first, read granulocytes explained. If you suspect your load has been too high for too long, spotting overtraining in your blood values is the logical next step, and tired despite good training covers the same question from the energy side.

References

1. Influence of training load on upper respiratory tract infection incidence and antigen-stimulated cytokine production. Scandinavian Journal of Medicine and Science in Sports. 2013. PMID 22151281.
2. Campbell JP, Turner JE. Debunking the Myth of Exercise-Induced Immune Suppression. Frontiers in Immunology. 2018. PMID 29713319.
3. Campbell JP, Turner JE. Can exercise affect immune function to increase susceptibility to infection? Exercise Immunology Review. 2020. PMID 32139352.
4. Gleeson M, et al. Exercise and the regulation of immune functions. Progress in Molecular Biology and Translational Science. 2015. PMID 26477922.
5. NHG. Dutch College of General Practitioners, respiratory infections in primary care. Accessed 2026.
6. RIVM. Respiratory infections, surveillance and seasonal pattern. Accessed 2026.
7. Thuisarts. I get frequent colds. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.